Many medicament delivery devices on the market have been developed for self administration of medicament, where one large group is medicament injection devices. Many of these injection devices have been provided with removable needle cover assemblies where the core cover is a needle sheath.
These sheaths are arranged to protect an injection needle before use in order to keep the needle sterile and also to protect the user from unintentional needle sticks. The sheaths are pushed onto the neck portion of a medicament container, such as a syringe or cartridge, during assembly of container, needle and sheath. The sheath is preferably of a resilient material, normally rubber, which ensures a tight grip and a good seal between the outer environment and the syringe. However this tight grip also means that it is relatively difficult to remove the sheath from the syringe in order to perform an injection.
Therefore a number of sheath removal devices have been developed, which are intended to aid the user in removing the sheath. Documents WO2010089589 and EP2021057 disclose different solutions to this problem.
WO2010089589 describes a device in which a housing of an autoinjector comprises an axially extending boss positioned to interfere with cam surfaces of axially extending fingers of a needle sheath removing means. During axial separation, the interference of the cam surfaces and the boss forces the fingers to flex radially inwards to grip a needle sheath of the autoinjector. This solution suffers the disadvantage of having the boss integrated with the housing, i.e. stationary, as the needle sheath moves axially in relation to the housing. If the fingers do not attain a good grip until the cam surfaces fully enter the narrow passage formed by the boss of the housing, the sheath may refuse to move and the fingers will slide. The sheath remover also risks getting stuck due to tolerance mismatch which may cause passage of the boss/housing to be too narrow for the cam surfaces to be able to enter it. Also, the proposed design of the autoinjector housing is very complicated to manufacture.
EP2021057 describes a cap for an injection device comprising a first part with a grip means for engaging the rear surface of a needle sheath at a neck portion of a medicament container. Upon assembly of the device the first part is attached to the front end of the injection device such that the grip means snap into engagement behind the needle sheath. A second part is subsequently attached to the first part to prevent the grip means from disengaging the rear surface of the sheath.
To grip behind the needle sheath is difficult since tolerances from assembling the containers and needle sheaths are not very accurate. If, during assembly of sheath, needle and container, the sheath is pushed far onto the neck portion of the container there may not be enough room for the fingers of the grip means to engage behind the sheath. Also, the grip is not very tight since it only depends on the grip means flexing radially behind the sheath and then being locked by the second part. There is no force to squeeze the sheath during removal. Therefore, such a solution would risk losing its grip on a soft rubber needle sheath.
An added disadvantage is that as the user tries to remove the cap and the sheath from the injection device he will have to overcome the friction between the cap and the device as well as the friction between the sheath and the needle at the same time. It may be difficult for users with weak hands, or for users with impaired manual dexterity, to overcome the accumulated force of the two friction interfaces.
Therefore, there is a need for a device that can securely remove a needle sheath from a needle regardless of the tolerances of the container or sheath. The device should also require little strength to remove and should be simple to manufacture.